Wind Chill Index - Wind chill is based on the rate of heat loss
from exposed skin caused by the combined effects of wind and cold. As the
wind speed increases, heat is carried away from the body at an accelerated
rate, driving down the body temperature. The wind chill temperature, an
"apparent" temperature, gives us a better estimate of how cold it really
feels outside. The measure of the rate of heat loss based on air
temperatures and wind speeds is not a temperature, but it allows us to
understand how quickly heat is lost to the wind.

As temperatures fall and the
wind howls, we begin hearing about the danger of "wind chill." The wind
chill index, shown above, combines the temperature and wind speed to tell
you how cold the wind makes it "feel." Even though the chill is given as a
temperature, it's not really a different kind of temperature. It's based on
polar research going back to the early 1940s and is meant to be a guide to
how much you should bundle up when going outside. Low wind-chill numbers
shouldn't keep you from going out; they should encourage you to dress
properly, however. Meteorologists and other scientists have raised
scientific concerns about the whole idea of wind chill.

The wind chill formula,
its development
The term "wind chill" goes back to the Antarctic explorer Paul A. Siple, who
coined it a 1939 dissertation, "Adaptation of the Explorer to the Climate of
Antarctica." During the 1940s Siple and Charles F. Passel conducted
experiments on the time needed to freeze water in a plastic cylinder that
was exposed to the elements. They found that the time depends on how warm
the water is at the beginning, the outside temperature and the wind speed.

T(wc) is the wind chill, V is in the wind speed
in statute miles per hour and T is the temperature in degrees Fahrenheit.

The formula to calculate a Celsius wind chill
using V as the wind speed in kilometers per hour and T in degrees Celsius
is:

T(wc) = 0.045(5.27V**0.5 + 10.45 - 0.28V) (T
- 33) + 33

Note: In
both formulas, ** means the following term is an exponent (i.e. 10**(0.5 )
means 10 to the 0.5 power, or the square root of V), - means to subtract, +
means to add. The standard rules of algebra apply.

Note: When
wind speeds are below 4 mph., the above formulas will give you a wind chill
that is higher than the actual temperature. When wind velocities are near
zero and you are standing still, your body heat warms the air near your
body. This warm air near your body provides some insulation from the colder
environment. As a result, it may actually feel warmer than the actual
temperature.